A direct reduction ironmaking process for the manufacture of agglomerative (including granular) metallic iron (reduced iron) from a mixture containing an iron-oxide source (hereinafter, also referred to as an “iron oxide-containing material”), for example, iron ore or iron oxide, and a carbon-containing reducing agent (hereinafter, also referred to as a “carbonaceous reducing agent”) has been developed. In this ironmaking process, compacts into which the mixture is formed are charged onto a hearth of a moving-bed heating furnace. The compacts are heated in the furnace by gas heat transfer and radiant heat with a heating burner to reduce iron oxide in the compacts with carbonaceous reducing agent. Subsequently, the resulting reduced iron is carburized, melted, and coalesced into agglomerates while being separated from by-product slag. Then the agglomerates are cooled and solidified to provide agglomerative metallic iron (reduced iron agglomerates).
Such an ironmaking process does not require a large-scale facility, such as a blast furnace, and has a high degree of flexibility in resources, for example, no need for coke; hence, the ironmaking process have recently been studied to achieve practical use. To perform it on an industrial scale, however, there are many problems regarding, for example, stable operation, safety, cost, the quality of granular iron (product), productivity to be solved.
In particular, in order to manufacture reduced iron agglomerates, it is desirable to improve the yield of large-grain reduced iron agglomerates and a reduction in manufacturing time. Regarding such a technique, for example, PTL 1 reports that “a method for manufacturing granular metallic iron includes heating a raw material that contains an iron oxide-containing material and a carbonaceous reductant to reduce a metal oxide in the raw material, further heating the resulting metal to melt the metal, and allowing the metal to coalesce to form a granular metal while being separated from a by-product slag component, in which a coalescence-promoting agent for the by-product slag is compounded in the raw material”.
In this technique, a large-grain granular metal should be manufactured in a high yield to some extent by compounding the coalescence-promoting agent (for example, fluorite). However, also in such a technique, the improvement effect is saturated, so further improvement of the effect is desired.
Regarding the quality of the reduced iron agglomerates, the granular iron manufactured by the foregoing ironmaking method is fed to an existing steelmaking facility and used as an iron source. Thus, the granular iron desirably has a low content of impurity elements, such as sulfur. As a technique therefor, for example, PTL 2 reports that “a method for manufacturing granular metallic iron having a low sulfur content includes charging a mixture that contains a metal oxide-containing substance and a carbonaceous reductant onto a hearth of a moving-bed heating furnace, heating the mixture to reduce iron oxide in the mixture with the carbonaceous reductant, allowing the metallic iron formed to coalesce into granules while the metallic iron is separated from a by-product slag, and solidifying the granules by cooling, in which the amounts of CaO, MgO, and SiO2-containing substances in the mixture are adjusted in such a manner that the basicity of slag components, i.e., (CaO+MgO)/SiO2, is in the range of 1.2 to 2.3 and that the content of MgO (MgO) in the components contained in the slag is in the range of 5% to 13%, determined from the contents of CaO, MgO, and SiO2 in the mixture”.
In this technique, a MgO-containing substance (for example, dolomite ore) is added to the mixture to adjust the slag components, thereby providing granular metallic iron having a low sulfur content. Also in this technique, the improvement effect is saturated, so further improvement of the effect is desired.
Note that the coalescence-promoting agent, such as fluorite, and the MgO-containing substance, such as dolomite ore, are both commonly used as melting-point-adjusting agents.